Page 247 - Plant-Based Remediation Processes

240 P. Kotrba

have an inherent ability to accumulate high concentrations of metals in the above-
ground biomass (Kabata-Pendias 2011). Of particular interest are species, referred to
as hyperaccumulators, that are able to accumulate in their shoots more than two and
up to four orders of magnitude higher concentrations of heavy metals than other
adjacent plants (Brooks 1998; Reeves 2006; Verbruggen et al. 2009; Kra ¨mer 2010).
The term hyperaccumulation was coined by Jaffre et al. (1976) who reported an
extreme phenotype of Sebertia acumunata. This species produces latex containing
1
up to 26 g Ni kg , probably the most extreme metal concentration reported in plants
to date. Currently, the accepted concentration criterions in shoot tissues of
hyperaccumulators on a dry-weight basis are >0.1 wt% for most metals, except,
for example, for zinc (>1 wt%), cadmium (>0.01 wt%), or gold (>0.0001 wt%)
(Baker et al. 2000). About 500 plant species of 34 families (0.2 % of angiosperms)
worldwide have been identified as hyperaccumulators of heavy metals (Co, Ni, Cu,
Zn, Cd, and Pb), metalloids (As), and nonmetals (Se) as well. With few exceptions,
among them Zn- and Cd-hyperaccumulating Sedum alfredii (Crassulaceae) or
Co- and Cu-hyperaccumulating Aeollanthus subacaulis (Lamiaceae), the plant
families most strongly represented are the Brassicaceae (in particular Alyssum
spp., Thlaspi/Noccaea spp., and Arabidopsis halleri), Euphorbiaceae, Asteraceae,
Flacourtiaceae, Buxaceae, and Rubiaceae (Reeves 2006). Regrettably, the use of
hyperaccumulators for large-scale phytoextraction is severely limited because of
their slow growth, low biomass, and often tight association with a specific habitat
and lack of good agronomic characteristics (Cunningham et al. 1995; Chaney et al.
2005; Meyer and Verbruggen 2012).
For phytoremediation-based reclamation of metalliferous soils to be successful,
plants should produce high biomass and accumulate and tolerate in their shoots high
levels of toxic metal species (Pilon-Smits 2005; Bhargava et al. 2012). The focus
for enhanced phytoremediation of soil metals is thus to use eligible plants with
higher biomass and well-established agriculture. Common high-biomass crop
plants or fast growing trees, such as poplar or willow, can be triggered to accumu-
late high amount of metals by enhancing the mobility of metal from the roots to the
green parts of the plant by adding mobilizing agents when the crop had reached
its maximum biomass (Huang and Cunningham 1996; Blaylock et al. 1997;Le
Cooper et al. 1999; Chen et al. 2004; Chaney et al. 2007). Though this approach
results in decontamination of soil it involves chemical intervention to the soil,
thereby causing secondary pollution. A suggested step forward for making
phytoremediation a viable technology is enhancing the metal accumulation and
tolerance by overexpressing in transgenic plants the genes involved in homeostasis,
metabolism, uptake, and translocation of the toxic elements. This chapter examines
suitable targets, outcomes, and prospects of transgenic plant research towards
upgraded phytoremediation plants.